The present invention relates to a printed circuit board and electronic equipment using the board. More specifically, the present invention relates to a printed circuit board that is favorable for achieving a high-speed signal interface requiring precise impedance matching, and electronic equipment using the printed circuit board.
Recently, in computer systems such as a personal computer, the speed of CPU and memory devices has increased to improve the overall system performance. The memory device used as the main storage has shifted from DRAM to DRAM with an EDO mode or a synchronous DRAM, and recently high-speed memory devices such as Rambus DRAM have been employed. A clock speed of CPU beyond 1 GHz has also recently been developed.
In accordance with the advances in speed of devices, the cycle time of signal transfer on the system board is being shortened year by year, necessitating the design of high-speed buses. When such a high-speed bus is used, precise impedance matching is required in order to prevent problems caused by reflection of transmitted signals.
When a signal is transmitted at high speed, a terminator (termination resistor) is often provided at the termination of a signal line in order to reduce the influence of reflection of the signal to a minimum. If the impedance of the signal line and that of the termination resistor are matched, reflection of the signal can be prevented and thus distortion is avoided.
In actual printed circuit boards, however, a power supply plane for supplying power to a signal line through a termination resistor has an impedance. Thus, the impedances of the termination resistor and the power supply plane are added to that of the signal line. Even if a termination resistor of the same impedance of that of the signal line itself, e.g., 50xcexa9, is connected to the signal line, precise impedance matching is still difficult to obtain.
An object of the present invention is to provide a printed circuit board capable of performing more precise impedance matching, and electronic equipment using the printed circuit board.
To attain the above object, a printed circuit board according to a first aspect of the present invention comprises: a ground layer; an insulation layer provided on the ground layer, including a first region having a first structure and a second region having a second structure; a first conductive pattern provided on the first region of the insulation layer; and a second conductive pattern provided on the second region of the insulation layer; wherein the second structure has an effect to reduce an impedance of the second conductive pattern, the effect is greater than an effect of the first structure to reduce an impedance of the first conductive pattern.
In the printed circuit board so constituted, the second conductive pattern provided on the second region of the insulation layer has an impedance which is lower than that of the first conductive pattern provided on the first region of the insulation layer. Thus, the impedance of the power supply plane to be formed of the second conductive pattern can be lowered, and the impedance of the termination resistor can easily be matched with that of the signal line. Consequently, precise impedance matching can be achieved.
Preferably, the insulation layer of the second region can be thinner than the insulation layer of the first region thereof, and the insulation layer can be constituted of two types of insulation members having different dielectric constants such that the dielectric constant of the insulation layer of the second region is higher than that of the insulation layer of the first region.
Moreover, the power supply plane can be coated with a dielectric material. Since, therefore, the area of contact between the power supply plane and the insulation layer can be increased, the impedance of the power supply plane can be lowered. It is preferable that marking paint be used as the dielectric material which coats the power supply plane. The marking paint is used to a mark the printed circuit board with the model numbers of electronic components and its relative dielectric constant is about 3. By using the marking paint as a dielectric material for coating the power supply plane, the impedance of the power supply plane can easily be lowered without preparing any special material.
According to a second aspect of the present invention, a printed circuit board in which a wiring layer is formed on a ground layer with a first insulation layer interposed therebetween, comprises: a first power supply plane provided on the wiring layer, for feeding power to a signal line formed on the wiring layer through a termination resistor connected to the signal line; a first via hole formed through the first insulation layer, for electrically connecting the first power supply plane and the ground layer; a second power supply plane provided under the ground layer with a second insulation layer interposed therebetween; and a second via hole formed through the first insulation layer, the ground layer, and the second insulation layer, for electrically connecting the first power supply plane and the second power supply plane.
In the printed circuit board so constituted, the second power supply plane is provided under the ground layer with the second insulation layer interposed therebetween, and thus the ground layer is sandwiched between the first and second power supply planes with the first and second insulation layers interposed therebetween. The capacitor including the first power supply plane, first insulation layer and ground layer is therefore connected in parallel with the capacitor including the first power supply plane, first insulation layer and ground layer, and the dielectric constant of the insulation layer, that exercises an influence on the impedance of the first power supply plane, can be lowered in substance. Consequently, the impedance of the first power supply can be lowered and the impedance of the termination resistor can easily be matched with that of the signal line.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.